The invention relates to an improved method and apparatus for separating two or more substances with different freezing points from a solution of a mixture of such substances using freeze concentration.
Concentrating aqueous beverages for storage, transportation and sale has a number of substantial advantages and is being used more and more widely for an increasing variety of beverages. For some products, for example, coffee and tea, the purpose of concentration is to produce a product which is convenient for the consumer to use. For other products, such as wine, milk, beer, vinegar and the like, the greatest advantage may lie in reducing the bulk of material, and thus reducing the expense of storage and transportation.
Concentration of such aqueous beverages can be done in one of three ways--evaporation, freeze concentration, or reverse osmosis. In evaporation techniques, the beverage is heated or steam is passed therethrough to remove the water by evaporation. In freeze concentration techniques, a slurry of ice is formed in the beverage and the ice then separated from the resulting concentrated liquor. One of the drawbacks to evaporation techniques is that many of the subtle flavor components of aqueous beverages are very volatile and readily escape during evaporation. This difficulty can in part be overcome by stripping many of these components before evaporation and returning them to the concentrated beverage thereafter. However, some degradation in flavor seems to be inevitable with evaporation techniques. Reverse osmosis is non-selective and flavor components are lost making it unsatisfactory for concentration of aqueous beverages.
Freeze concentrated products do not suffer from degradation since retention of flavor components is almost 100%. The main drawbacks in the past of freeze concentration processes have been expense and insufficient volume of operation.
A number of advances in recent years have made freeze concentration competitive with evaporation techniques and, indeed, for many applications clearly superior over this competing technology. In a typical freeze concentration system a slurry of tiny ice crystals is grown in a scrape surface heat exchanger or the like and continuously supplied to a recrystallization vessel in which the crystals grow. The patent to Thijssen et al U.S. Pat. No. 4,004,886 describes a process and apparatus of this general type in which a filter is used in the recirculation path to the scrape surface heat exchanger so that crystals are substantially prevented from leaving the recrystallizing vessel. Almost all the crystals in the recrystallizer melt and reform on a very few small seed crystals to produce a crystal slurry having relatively uniformsized crystals therein which slurry is continuously removed from the recrystallizer as a crystal suspension. Not only is residence time in the system substantially reduced and, therefore, the efficiency of the system improved, but the resulting uniform-sized seed crystals can be easily separated in a wash column, for example as described in Thijssen U.S. Pat. No. 3,872,009.
Additional economies are achieved by multistage systems using several sets of scrape surface heat exchangers and recrystallizing vessels connected in series as taught in U.S. Pat. No. 4,316,368, the disclosure of which is hereby incorporated by reference. The slurry of ice crystals drawn from one recrystallizing vessel is supplied to the next where the crystals continue to grow larger and larger. Concentrated mother liquor is moved in the opposite direction and the larger crystals eventually passed to a wash column. Some of the crystals are passed in the same direction as the mother liquor so that crystals in that preceding recrystallizing vessel melt and reform on the larger crystals. Thus, crystal growth takes place on crystals which already have grown large and can take place in a less concentrated solution. Residence time is decreased, capacity is increased and the efficiency of freeze concentration is substantially improved.
U.S. Pat. No. 4,406,679, the disclosure of which is hereby incorporated by reference, describes a specific freeze concentration system in which a slurry of crystals is formed in a freeze crystallizer and supplied to a so-called gradient column where the crystals are separated from the concentrated mother liquor and transferred to a dilute mother liquor. According to the patent, some crystal growth may also take place in the gradient column. The resultant dilute slurry is transferred to a holding tank in which the residence time is said to permit significant growth in crystal size. The dilute slurry with the larger crystals is then supplied to a purifier, more particularly a wash column, where the slurry is separated into mother liquor and oure crystals. The mother liquor is returned to the gradient column.
The present invention relates to improvements in the system described in said U.S. Pat. No. 4,406,679. In a first improvement, crystals from the gradient column and preferably crystals which have grown in the holding tank are fed back to the input of the gradient column and mixed with the concentrated slurry which is continuously fed thereto. As explained above, the smaller crystals in the concentrated slurry will dissolve and reform on the larger crystals fed back to the input from the holding tank or the output of the gradient column. The result will be improved efficiency of operation and a more uniform size of crystals, leading to more satisfactory operation of the wash column.
In a second improvement a plurality of units as described in U.S. Pat. No. 4,406,679 are connected in series for countercurrent operation. Gradient columns are employed to separate the ice from the slurry between stages.
Other objects and purposes of the invention will be clear from the following detailed description of the drawing.